The importance of hydrogen as a fuel source for fuel cells and the like, serves to emphasize the various processes used to provide hydrogen. Many of those processes rely on the reformation and/or refinement of various hydrocarbon feedstocks, such as petroleum derivatives, methane, coal-derived fuels and the like. The reformation of these feedstocks by such processes as catalytic steam reforming, autothermal reforming, or catalytic partial oxidation yield reformate, a mixture of hydrogen and carbon monoxide. Similarly the gasification by various processes of a wide variety of carbonaceous feeds like, but not limited to, bituminous coal, lignite, and petroleum resid or coke yield mixtures of hydrogen and carbon monoxide often containing sulfur compounds with an H to C ratio reflecting the H to C ratio in the feed and the amount of steam (if any) used in the gasification process. The further refinement as by the water gas shift (WGS) reaction to provide high purity hydrogen, requires the use of a catalyst that is both active with respect to the WGS reaction and is tolerant to the sulfur present from the feedstock during the process.
Various metal oxides have found use in chemically reactive systems as catalysts, supports for catalysts, gettering agents and the like. In those usages, their chemical characteristics and morphologies may be important, as well as their ease and economy of manufacture. One area of usage that is of particular interest is in the production of hydrogen from carbonaceous fuels. Carbonaceous fuels are those typically containing at least 0.9 hydrogen per unit of carbon, and may include hetero atoms such as O, N, and/or S. Hydrogen production from these carbonaceous fuels typically requires the conversion of carbon monoxide and water into carbon dioxide and hydrogen through the water gas shift (WGS) reaction. Industrially, iron-chrome catalysts, often promoted, are used as high temperature shift catalysts, and copper-zinc oxide catalysts, often containing alumina and other products, are effective low temperature shift catalysts. These catalysts are less desirable for use in fuel processing systems because they require careful reductive activation and can be irreversibly damaged by air after activation.
Recent studies of automotive exhaust gas “three-way” catalysts (TWC) have described the effectiveness of a component of such catalysts, that being noble metal on cerium oxide, or “ceria” (CeO2), for the water gas shift reaction because of its particular oxygen storing capacity (OSC). Indeed, the ceria may even act as a “co-catalyst” with the noble metal loading in that it, under reducing conditions, acts in concert with the noble metal, providing oxygen from the CeO2 lattice to the noble metal surface to oxidize carbon monoxide and/or hydrocarbons adsorbed and activated on the surface. This is described in greater detail in an article entitled “Studies of the water-gas-shift reaction on ceria-supported Pt, Pd, and Rh: implications for oxygen storage properties”, by T. Bunluesin, et al, in Applied Catalysis B: Environmental 15 (1998) at pages 107-114. Another, possibly parallel or possibly alternative, mechanism for the water gas shift reaction over a CeO2 support lattice is described in an article entitled “Reactant-Promoted Reaction Mechanism for Water-Gas Shift Reaction on Rh-Doped CeO2” by T. Shido, et al, in Journal of Catalysis, 141, (1993) at pages 71-81, in which formate is identified as a WGS intermediate produced from CO and surface OH groups.
In many cases the ceria component of these catalysts is not pure ceria, but cerium oxide mixed with zirconium oxide and optionally, other oxides such as rare earth oxides. It has been determined that the reduction/oxidation (redox) behavior of the cerium oxide is enhanced by the presence of ZrO2 and/or selected dopants. Robustness at high temperatures is an essential property of TWC's, and thus, such catalysts do not typically have either sustainable high surface areas, i.e., greater than 100 m2/g, or high metal dispersion (very small metal crystallites), even though such features are generally recognized as desirable in other, lower temperature, catalytic applications.
For mixed-metal oxides that are to be used as co-catalysts, referred to herein as “supports” and which comprise cerium oxide and zirconium and/or hafnium oxide, it is generally desirable that they possess a cubic structure. The cubic structure is generally associated with greater oxygen mobility, and therefore greater catalytic activity. Moreover, the zirconium and/or hafnium provide thermal stability, and thus contribute to the thermal stability and life of a catalyst. Yashima et al., in an article entitled “Diffusionless Tetragonal-Cubic Transformation Temperature in Zirconia Solid Solution” in Journal of American Ceramic Society, 76 [11], 1993, pages 2865-2868, have shown that cubic ceria undergoes a phase transition to tetragonal when doping levels of zirconia are at or above 20 atomic percent. They suggest that above 20 percent zirconia, the oxygen anion lattice distorts into a tetragonal phase, while the cerium and zirconium cations remain in a cubic lattice structure, creating a non-cubic, metastable, pseudo-tetragonal phase lattice. Traditionally, powder X-ray diffraction (PXRD) is used to identify the structure and symmetry of such phases. However, in the case of ceria-zirconia oxides with very small crystallite sizes (i.e., less than 3 nm), the PXRD signal exhibits broadened peaks. Additionally, the signal produced by the oxygen atoms, which is a function of atomic weight, is drowned out by the intense signal produced by the cerium and zirconium cations. Thus any tetragonal distortion, caused by the oxygen atoms shifting in the lattice, goes unnoticed in a PXRD pattern and the resulting pattern appears cubic. In such cases, Raman spectroscopy and X-ray absorption fine structure (EXAFS) can be employed to observe such phase transitions. Yashima et al. have published Raman spectroscopy and EXAFS studies in support of the position taken above. Vlaic et al., in an article entitled “Relationship between the Zirconia-Promoted Reduction in the Rh-Loaded Ce0.5Zr0.5O2 Mixed Oxide and the Zr—O Local Structure” in Journal of Catalysis, 168, (1997) pages 386-392, have shown similar results for a phase transition at 50% zirconia, as determined by Raman spectroscopy and EXAFS.
A variety of synthesis techniques have been used to provide ceria-zirconia mixed oxide materials. These techniques include conventional co-precipitation, homogeneous coprecipitation, the citrate process, and a variety of sol-gel techniques. However, as far as can be determined, the surface areas of the mixed metal oxides resulting from these techniques are typically less than about 130 m2/g. Liquid phase synthesis at relatively low temperatures is preferred, as it allows for the formation of metastable phases and offers the ability to control such properties as surface area, particle size, and pore structure. Typical solution routes have involved two steps, hydration and condensation. It has been generally accepted that the gel matrix formed upon hydration is amorphous and only forms a crystalline structure when the framework undergoes condensation. While hydration occurs at the moment the gelatinous phase is formed from solution, condensation has usually been expected to occur during the aging (maturing), drying and/or calcinations steps. For many mixed metal oxide systems, the detailed conditions under which these steps (such as aging) occur are, and have been, critical parameters in determining the properties of the final product. Thus, a time consuming step such as aging has been essential.
Surface areas as great as 235 m2/g for such materials have been reported by D Terribile, et al, in an article entitled “The preparation of high surface area CeO2-ZrO2 mixed oxides by a surfactant-assisted approach” appearing in Catalysis Today 43 (1998) at pages 79-88, however, the process for their production is complex, sensitive, and time-consuming. The process for making these oxides requires the use of a surfactant and a lengthy aging, or maturing, interval of about 90 hours at 90° C. Moreover, the initial precipitate must be washed repeatedly with water and acetone to remove the free surfactant (cetyltrimethylammonium bromide) before the material can be calcined, thereby contributing to delays and possible other concerns. Still further, the mean particle sizes of these oxides appear to be at least 4-6 nm or more. The pore volume is stated to be about 0.66 cm3/g. This relatively large pore volume per gram is not consistent with that required for a ceria-based mixed metal oxide which, while thermally robust, should tend to maximize both the available surface area in a given reactor volume and the mass transfer characteristics of the overall structure as well as the appropriate reactivity of that surface area, as is desired in the applications under consideration. Assuming the density, D, of this material is about 6.64 g/cm3, the skeleton has a volume, VS, of 1/D, or about 0.15 cm3/g, such that the total volume, VT, of one gram of this material is the sum of the pore volume, PV, (0.66 cm3/gm) and the skeletal volume, VS, which equals about 0.81 cm3/gm. Hence, 235 m2/gm÷0.81 cm3/gm equals about 290 m2/cm3. Because of the relatively large pore volume, the surface area per unit volume of a material of such density has a reduced value that may not be viewed as optimal.
For use of a mixed-metal oxide in a catalyst application, it is required to be loaded with a metal, such as a noble metal, providing good catalytic activity to the media being processed. While noble metals such as platinum have provided good catalytic activity, it is always desirable to improve the activity, cost, and/or durability of such catalyst metal loadings.
Sulfur tolerance is a critical requirement of the catalyst in promoting durability and avoiding a fast deactivation mechanism associated with sulfate and sulfide formation. This is especially true for relatively large, high pressure installations like coal-to-hydrogen plants. Typically a dry, oxygen-blown bituminous gasifier will produce gas that, after precleaning and before the addition of steam, has the approximate composition of: 34% H2, 61% CO, 2% CO2, 3% others; and 200 ppmv sulfur as H2S and COS. Steam is added before the WGS step to achieve 3.1H2O/(CO+CO2) ratio. The development of advanced membrane WGS reactors is focusing particularly on membrane-type WGS reactors designed to yield high purity hydrogen permeate and carbon dioxide rich retentate. These membrane-type WGS reactors require catalysts operating across the temperature range of about 300-450° C., at pressures up to 50 atm or more of reformate gas, with a ˜3H2O/(CO+CO2) feed ratio and a feed sulfur partial pressure (expressed as H2) of about 5×10−3 atm (3.8 torr) at 50 atm (38,000 torr). This sulfur content, then, may be in the range of 1.8 to as much as 70 ppm for a 5-200 ppm feed gas without/before the addition of steam. For economical operation, the conversion rate in terms of moles of CO/mole Pt-sec over a Pt-alloy/mixed metal oxide catalyst needs to be at least 33% of its initial (250 hrs) lined out activity at about 400° C. after 45,000 hours of operation. In this way, if at 400° C. and after 45,000 hours of operation the activity of the catalyst remains at or above 33% of its initial activity, it is then possible to operate near 450° C. and retain nearly the 100% activity.
Until recently, the best commercially available water gas shift catalysts have been the above-mentioned Fe—Cr and Cu/ZnO. These catalysts, however, do not meet the above-stated criteria. Instead they exhibit relatively low volumetric activity, are pyrophoric, and significantly lack sulfur tolerance. However, recent developments have sought to overcome or at least improve upon, one or more of those limitations.
Accordingly, it is a primary object of the present disclosure to provide a catalyst support and catalyst that possess extended activity and durability in WGS reactions under conditions of significant sulfur presence, elevated operating temperatures, and/or elevated pressures associated with the carbon oxides.
It is a further object of the present disclosure to provide a method of making such catalyst support and catalyst.
It is a still further object of the present disclosure to provide the use of such catalyst support and catalyst in a water gas shift reaction.
These and other objects and advantages will be apparent herein.